Temperature compensated strain sensing apparatus

ABSTRACT

A strain sensing apparatus for use in the presence of thermal gradients has a strain gauge and resistance temperature gauge in a planar assembly bonded to, deposited on or diffused or implanted in a substrate surface. The apparatus can sense strain and temperature independently and can be used alone or with a plurality of installations in strain gauge based transducers or experimental stress analysis. The output from the temperature gauge can be used accurately to compensate for unwanted thermal effects in the strain gauge and to measure temperature at the strain gauge site.

[0001] This invention relates to a strain sensing apparatus orinstallation, and in particular to such an apparatus or installation foruse in the presence of thermal gradients.

[0002] Strain gauges are well known devices for measuring mechanicalstrain in engineering materials and they are commonly used inexperimental stress analysis and in the manufacture of many kinds oftransducer, where the measured parameter is inferred from the strain ina flexural element by strain gauges bonded to it. Pressure transducers,torque cells, load cells and accelerometers are typical examples.Unfortunately most strain gauges are sensitive to strain andtemperature, in roughly equal parts, and there are known techniques forcompensating strain gauge installations for temperature effects inexistence. The most common method is to employ at least two straingauges in a Wheatstone bridge circuit so arranged that changes inresistance due to strain are reinforced and changes in resistance due totemperature are cancelled. For higher accuracy, the bridge circuit maybe combined with a temperature-sensing device, where the latter is usedto compensate for secondary errors in the former arising from mismatchof the thermal parameters of the strain gauges and materials within thetransducer.

[0003] However, these techniques are only effective in isothermalconditions. This is particularly the case when the temperature-sensingdevice is not in the same position spatially as the strain gauge(s).Such techniques are sufficient for many applications where theisothermal assumption is appropriate or low accuracy is adequate. But inapplications where the installation is subject to thermal transients,then quite large zero and span errors can occur during the transientperiod, caused by thermal gradients in the transducer body. Where thetransient is the period of interest the above techniques are notsuitable. A typical application of this type is the measurement ofengine torque using a torque cell built into the clutch shaft of aracing car during heavy acceleration, when the temperature of the clutchand surrounding components, including the torque cell, rises rapidly.

[0004] Furthermore, it is increasingly common in the production of highaccuracy transducers to compensate for thermal and other errors usingdigital techniques. For thermal errors the transducer is calibrated atseveral points in the relevant thermal spectrum and the output of anintegral temperature-sensing device recorded. Compensation is achievedby storing empirical coefficients, acquired during calibration, indigital memory built into the transducer. In service the coefficientsare recalled by built-in electronics in accord with the integraltemperature sensing device output and are used to correct the transduceroutput for temperature effects on zero and span. The empiricalcoefficients are typically acquired under almost isothermal conditionsin the factory and the transducer must be used in similar conditions orthe stated accuracy will not be maintained. This limits the applicationsfor high accuracy transducers of this type.

[0005] In order to address one or more of the above problems, thepresent invention provides, in a first aspect, a strain sensingapparatus having a strain gauge having a strain sensing area, and atemperature gauge having a temperature sensing area in thermal contactwith the strain sensing area, wherein one of the strain sensing area andthe temperature sensing area overlies the other.

[0006] It is therefore possible substantially to eliminate (forpractical purposes) a time lag between the temperature of the straingauge and the temperature of the temperature gauge. Thus, thetemperature of the strain gauge (and preferably of the whole of thestrain sensing area) can be accurately monitored by the temperaturegauge.

[0007] The term “overlie” can be construed here as “overlap”. It is tobe understood that the orientation of the apparatus, and the relativeorder of the strain gauge and the temperature gauge do not affect theway in which the invention works.

[0008] Advantageous features of the temperature gauge include lowthermal mass, allowing the temperature gauge to track temperaturechanges in the strain gauge with little or no time lag. Typically, lowthermal mass can be achieved by making the temperature gauge (or atleast the temperature sensing part of the gauge) thin in one dimension,but with a high surface area. Preferably, the temperature gauge is aresistance temperature gauge, i.e. a temperature gauge which relies on achange in resistance of the sensing portion with temperature in order todetermine the temperature.

[0009] Preferably, the strain sensing area and the temperature sensingarea substantially match in size and are overlaid substantially tocoincide. This can enhance the temperature-tracking of the temperaturegauge with respect to the strain gauge.

[0010] Preferably, the strain gauge is bonded to the temperature gauge.Direct bonding (e.g. without an intermediate layer, apart from a bondinglayer, if required) may be preferable in order to ensure good thermalcontact between the strain sensing area and the temperature sensingarea. Typically, strain and/or temperature sensing gauges include alayer to which the sensing area is bonded. This layer may be adielectric material layer.

[0011] The apparatus may include a plurality of strain gauges, eachstrain gauge having a corresponding temperature gauge as set out in thisfirst aspect of the invention. Each strain gauge may be located at adifferent site in the apparatus. Some or all of the strain gauges may bearranged in, e.g., a Wheatstone bridge type circuit in order to, e.g.,compensate for thermal errors in isothermal and/or non-isothermal and/orthermal transient conditions.

[0012] The apparatus may be a transducer, e.g. for stress measurementsbased on strain measurements in a structure.

[0013] Embodiments of the invention may be used, for example, foraccurate measurement of thermal stress in a structure, induced by, e.g.,a thermal transient. Typically, this is done using one strain gauge withaccompanying resistance temperature gauge overlay (or underlay).Typically, the apparatus requires calibration under isothermalconditions. Embodiments of the invention may also be used to measure theaverage temperature at a site in a structure.

[0014] In applications where a Wheatstone bridge circuit is used onlyfor thermal compensation, as in the case of a pressure diaphragm, thefunction of the Wheatstone bridge circuit (or similarly functioningcircuit) can be replaced by, e.g., a single strain gauge withtemperature gauge overlay (or underlay), thus saving materials andlabour.

[0015] Preferably, the strain gauge has a strain sensing pattern and thetemperature gauge has a temperature sensing pattern of substantially thesame shape, the gauges being overlaid substantially to match thepatterns. Matching patterns in this way gives rise to a favourabletracking of the temperature of the strain sensing area by thetemperature sensing area.

[0016] Alternatively, the strain gauge may have a strain sensing patternand the temperature gauge has a temperature sensing pattern selected tocomplement the strain sensing pattern. In that case, the temperaturesensing pattern need not be substantially identical to the strainsensing pattern. The temperature sensing pattern may be chosen for asecondary function, e.g. a strain sensing function.

[0017] Preferably, the temperature sensing pattern is selected so that,in use, when subjected to a predetermined non-zero strain or to apredetermined non-zero strain format, the temperature gauge hassubstantially zero net strain output.

[0018] Typically, the strain gauge is formed on a substrate by selectivedeposition. Various deposition techniques may be used, e.g. thin filmtechniques combined with patterning techniques may be used. The straingauge may be formed before application to the substrate, e.g. in theform of a foil strain gauge.

[0019] Alternatively, the strain gauge may be formed on or in asubstrate by selective diffusion into the surface of the substrate. Inthat case, the strain gauge may be formed in the substrate (e.g. in asemiconducting substrate such as silicon) by selectively doping thestrain sensing pattern into the substrate. This can give rise to highlyaccurate patterns for the strain sensing area.

[0020] Preferably, the temperature gauge is formed before or after thestrain gauge by selective deposition onto the substrate or strain gauge.Again, various deposition techniques may be used, e.g. thin filmtechniques combined with patterning techniques may be used. Thetemperature gauge may be formed before application to the substrate,e.g. in the form of a foil temperature gauge. Usually, the temperaturegauge is applied over the strain gauge with respect to the substrate.

[0021] Alternatively, the temperature gauge may be formed before orafter the strain gauge by selective diffusion into the substrate or intoa layer overlying the strain gauge. In that case, the temperature gaugemay be formed in the substrate or in a layer covering the strain sensingarea (e.g. in a semiconducting substrate or layer such as silicon) byselectively doping the temperature sensing pattern into the substrate orlayer. This can give rise to highly accurate patterns for thetemperature sensing area which can therefore be matched accurately (interms of shape and/or spatial positioning) with the strain sensingpattern.

[0022] The strain gauge may be formed, at least in part, of a materialcomprising platinum and tungsten. Preferably, the material used is analloy of platinum and tungsten such as Pt92/w8. However, the materialfor the strain gauge may be any material which has a suitably high gaugefactor (GF), as explained in more detail below.

[0023] Certain strain gauge materials are known to have a high gaugefactor (GF) and are thus useful where strain levels are low ormeasurements are made in a noisy environment. Unfortunately thesematerials often have a high temperature coefficient of resistance (TCR)and may have other undesirable thermal effects and are only suitable fordynamic measurements as the normal level of non-isothermal effects on aWheatstone bridge produce large errors when used for staticmeasurements. Combining resistance temperature gauges as described abovewith strain gauges made from these materials can allow their use forstatic measurements in non-isothermal conditions. A typical material ofthis type is platinum/tungsten alloy (such as Pt92/W8), which has a GFmore than twice that of most metals. Disadvantageously it has almost tentimes the problematic thermal effects, which has made this materialnon-desirable in the past for non-isothermal strain measurements.

[0024] Alternatively, the strain gauge may be formed, at least in part,of a material comprising silicon. Preferably, the material used is adoped silicon, doped in order to provide a preferential conducting paththrough the semiconducting silicon. A typical dopant suitable for thispurpose is boron.

[0025] Silicon strain gauges, diffused or implanted into the surface ofa Silicon wafer, have the advantage that areas of the wafer can beconfigured as mechanical flexures and, using standard integrated circuittechnologies, a pressure diaphragm or accelerometer beam complete withappropriately sited strain gauges, connections and passivation can beproduced in a small area of the wafer. Using the benefits of batchprocessing around 1000 units can be produced on one wafer which is verycost effective and automotive applications include transducers forengine and airbag controllers. It is recognised that this type oftransducer, whilst reliable and stable, is subject to thermal errors andits use is restricted to low cost/low accuracy applications. However,when used in combination with the present invention, this type oftransducer can be improved in its current applications and its useextended to other applications requiring accuracy in non-isothermalconditions. This can be achieved by depositing temperature gauges onSilicon strain gauges at the time of manufacture and incorporating thetemperature signals generated by them in a temperature compensationscheme in accord with the present invention.

[0026] Typically, the temperature gauge is formed, at least in part, ofa material comprising nickel or platinum. This is explained in moredetail below.

[0027] Normally, the resistance temperature gauge can be manufacturedusing the same plant and techniques as the strain gauge, and if desiredthe strain gauge manufacturer can carry out the overlay bonding ordepositing at the time of manufacture, to produce a single assembly ofstrain gauge and temperature gauge for the convenience of the user. Inthe case of deposited or implanted gauges the assembly of the gauges andbonding to the flexure can be achieved at the time of manufacture.

[0028] In a second aspect, the present invention provides an apparatusfor outputting a temperature-compensated strain measurement, including a(or more than one) strain sensing apparatus according to the firstaspect, and means for correcting the output from the strain gauge ofsaid strain sensing apparatus using the output from the temperaturegauge of said strain sensing apparatus.

[0029] Preferred features described with respect to the first aspect maybe incorporated in this second aspect. In particular, the circuitrydescribed with respect to the first aspect may be used with the secondaspect.

[0030] In another aspect, the present invention provides a method ofmeasuring strain using a strain sensing apparatus according to the firstor second aspect, wherein the output from the strain gauge is correctedto give a temperature-compensated strain output according to the outputfrom the temperature gauge.

[0031] Preferably, the strain is measured during a thermal transientapplied to the strain sensing apparatus.

[0032] Several embodiments of the invention will now be described by wayof example only, with reference to the accompanying drawings in which:

[0033]FIG. 1 shows a schematic view of an apparatus according to oneembodiment of the invention, having a single strain gauge andcompensating temperature gauge.

[0034]FIG. 2 shows an exploded view of the apparatus of FIG. 1.

[0035]FIG. 3 shows a schematic, exploded view of an apparatus accordingto another embodiment of the invention, having a dual strain gaugepattern for measuring compressive strain, to be used, for example, incolumn load cells, with a companion temperature gauge.

[0036]FIG. 4 shows a schematic, exploded view of an apparatus accordingto another embodiment of the invention, having a strain gauge patternfor measuring shear strain, to be used, for example, in torque cells andload cells, with a compensating temperature gauge overlay.

[0037]FIG. 5 shows a schematic, exploded view of an apparatus accordingto another embodiment of the invention, having as strain gauge patternfor use with a pressure transducer diaphragm, with a compensatingtemperature gauge overlay.

[0038]FIG. 6 shows a circuit for use with or in an embodiment of theinvention, wherein compensation is achieved by analogue techniques.

[0039]FIG. 7 shows a circuit for use with or in an embodiment of theinvention, wherein compensation is achieved using digital techniques.

[0040]FIG. 8 shows a schematic, exploded view of an apparatus accordingto another embodiment of the invention, having a silicon pressure sensordiaphragm wherein single strain and temperature sensing elements areemployed, these having been manufactured using silicon integratedcircuit technology.

[0041]FIG. 9 shows the apparatus of FIG. 8, viewed from the underside toshow the diaphragm forming cavity.

[0042]FIG. 10 shows a schematic, exploded view of an apparatus accordingto another embodiment of the invention, having a pressure sensordiaphragm wherein single strain and temperature sensing elements areemployed, these having been manufactured using thin film depositiontechniques.

[0043] Referring first to FIG. 1, an apparatus or installationconsisting of a substrate 1, a dielectric layer 2, a strain sensing grid3, manufactured from a suitable metal alloy such as platinum/tungsten(Pt92/W8), another dielectric layer 4 and a temperature sensing grid 5,manufactured from suitable metal such as nickel. The substrate 1 and thelayers 2, 3, 4 and 5 are securely bonded together using adhesive,deposition, implantation or diffusion techniques and it is usual to sealthe assembly with a overall protective layer appropriate to theapplication. The temperature gauge dielectric layer 4 is preferablymanufactured from the same material as the strain gauge dielectric layer2 and should be as thin as practical. The temperature sensing gridpattern 5 and the strain sensing grid pattern 3 are identical andcoincident in the plane of the substrate in order to provide the besttemperature tracking. Electrical connection to the gauges can be made byfine wire bonding or soldering at solder pad 6.

[0044]FIG. 2 is an exploded view of the assembly of FIG. 1 showing thesub-assemblies of the strain gauge and temperature gauge separated forclarity.

[0045]FIG. 3 shows a dual strain gauge 82 which can be used to measurecompressive strain (e.g. planar) in column type load cells withcompanion resistance temperature gauges 84,86. The grid patterns of thetemperature gauges 84,86 are designed to exploit Poisson's effect in thesubstrate so that when subjected to a strain on the longitudinal axis ofthe strain gauge grid the net change in resistance is zero. This isuseful if the strain sensing ability of the temperature sensing alloyresults in a significant error in temperature sensed when the substrateis strained. The grid pattern can be designed on a theoretical basis andcorrected empirically to achieve zero resistance change when strainedfor a given substrate, such as stainless steel. The grid design willgenerally vary with Poisson's ratio and once established for a materialshould remain constant. There are various patterns that can achieve thisand the pattern shown is one of many. The pattern area and shape should,preferably, substantially mimic the strain gauge 82 as shown. In somecases, where tensile and compressive strains exist in close proximity,such as a pressure diaphragm or shear beam, the temperature gauge gridcan be designed to exploit this, as shown in FIG. 5, and achieve zeronet strain output.

[0046]FIG. 4 shows a strain gauge 92 which can be used for sensing shearstrain in load or torque cells with a temperature gauge overlaid anddesigned according to the zero strain rules.

[0047]FIG. 5 shows a strain gauge 7 and temperature gauge 8 designed fora circular pressure diaphragm to give an embodiment of the inventionwhich exploits the strain distribution in the diaphragm (not shown). Thestrain gauge 7 senses tangential strain in the diaphragm which is zeroat the edge of the diaphragm and maximum tensile at the centre. Thetemperature gauge 8 covers the same area but senses radial strain, whichvaries from maximum compressive at the edge of the diaphragm to maximumtensile at the centre. The length and position of the radial pattern isdesigned so that the net radial strain sensed is zero.

[0048]FIG. 6 shows a circuit employing four strain gauges (SGa, SGb, SGcand SGd) and companion temperature gauge overlays (TGa, TGb, TGc, TGd)wired as two Wheatstone bridges connected in series. The temperaturegauge bridge is wired as a mirror image of the strain gauge bridge sothat the outputs due to temperature imbalance on the substrate are inopposition. The resistors Rcomp are set empirically or by calculation sothat the output at the strain bridge due to thermal imbalance is zero.Under isothermal conditions there will be no output from either bridgein zero strain condition; when the substrate is strained only the strainbridge will output. It is also possible to wire the bridges in paralleland many permutations for compensation are possible using passive andactive components.

[0049] When active components are used it is possible to dispense withthe Wheatstone bridge circuit and accomplish temperature compensationwith only one strain gauge and its temperature gauge companion. A systemfor achieving this is shown in FIG. 7. A precision constant currentsource CON I biases TG and SG and the signals from same arealternatively multiplexed by MUX before analogue to digital conversionby ADC. This enables the signal data to be processed by a microprocessorMICRO PROC and after processing the data is converted to an analoguesignal by the digital to analogue converter DAC so that common analogueinstruments can be used to read the corrected data. The system iscalibrated empirically at several points in the temperature/strainenvelope in a test rig capable of applying strain and temperaturesimultaneously or independently to the sensing substrate or flexure andthe test point constants stored in non-volatile memory EEPROM. Inservice the signals from TG and SG are monitored and the strain signalis corrected for temperature effects by the microprocessor using thetest constants stored in EEPROM prior to analogue conversion.

[0050] The ability to employ a single strain gauge/temperature gaugepair as a compensated measuring solution is particularly useful in thecase of silicon pressure transducers. The manufacture of siliconintegrated circuits does not lend itself to the production of precisesilicon resistors and using four resistors in a Wheatstone bridgecircuit does not necessarily guarantee sufficient precision. The singlestrain gauge/temperature gauge pair does not require precision in thecomponents, relying instead on stability, tracking and calibration foraccuracy. The solution is also simpler, more reliable and should lead tohigher yields, which converts to lower costs with improved performance.Referring to FIG. 8, the silicon die 9 has strain sensing resistor 12diffused or implanted into the surface. The shape and position of theresistor is designed to sense tangential strain in the circulardiaphragm 15 (formed in the die by the cavity shown in FIG. 9) whenpressure is applied to it. A thermal oxide passivation layer 10 isformed over the strain sensing resistor and two small apertures 14aligned with the strain resistor terminals are etched in it usingestablished techniques utilising photo configurable sacrificial layers.The temperature sensing resistor 11 is deposited on layer 10 and isconfigured, using photo techniques as above, so that the sensing ring isexactly the same shape, size and position as the strain sensing resistorsensing ring below it. The temperature sensing resistor is manufacturedfrom a suitable material such as nickel or platinum. The terminations 13of the resistors can be formed by photo configuring this layer in thesame operation as shown in FIG. 8.

[0051] A similar assembly can be manufactured using thin film techniquesas shown in FIG. 10 where the substrate 16 can be manufactured from amaterial able to resist chemical attack from the pressure medium such asstainless steel. The circular diaphragm can be formed in the substratein a similar manner to that shown in FIG. 9. The dielectric layer 17 isformed on the substrate and the strain sensing resistor 18 is depositedon it and configured either by etching or depositing through a mask. Awide choice of suitable materials for the strain sensing resistor isavailable by virtue of the sputtering process. One such material isplatinum tungsten alloy (Pt92/W8). The second dielectric layer 19 isformed over the sensing resistor and photo configured by wellestablished techniques to provide the termination apertures 14. Thetemperature sensing resistor 20 is deposited on the dielectric layer andconfigured using the same technique as that used to configure the strainsensing resistor so that the sensing ring is exactly the same shape,size and position as the strain sensing resistor ring below it. Theterminations 13 of the resistors can be formed when this layer isconfigured. Suitable materials for the temperature sensing layer includenickel or platinum.

1. A strain sensing apparatus having: a strain gauge having a layer towhich a strain sensing area is bonded, a temperature gauge having alayer to which a temperature sensing area is bonded, the temperaturesensing area being in thermal contact with the strain sensing area,wherein one of the strain sensing area and the temperature sensing areaoverlies the other, the temperature gauge being directly bonded to thestrain gauge.
 2. A strain sensing apparatus according to claim 1 whereinthe layer to which the strain sensing area is bonded and/or the layer towhich the temperature sensing area is bonded is a dielectric materiallayer.
 3. A strain sensing apparatus according to claim 1 or claim 2wherein the strain sensing area and the temperature sensing areasubstantially match in size and are overlaid substantially to coincide.4. A strain sensing apparatus according to any one of claims 1 to 3wherein the strain gauge has a strain sensing pattern and thetemperature gauge has a temperature sensing pattern of substantially thesame shape, the gauges being overlaid substantially to match thepatterns.
 5. A strain sensing apparatus according to any one of claims 1to 3 wherein the strain gauge has a strain sensing pattern and thetemperature gauge has a temperature sensing pattern selected tocomplement the strain sensing pattern.
 6. A strain sensing apparatusaccording to claim 5 wherein the temperature sensing pattern is selectedso that, in use, when subjected to a predetermined non-zero strain or toa predetermined non-zero strain format, the temperature gauge hassubstantially zero net strain output.
 7. A strain sensing apparatusaccording to any one of claims 1-6 wherein the strain gauge is formed ona substrate by selective deposition.
 8. A strain sensing apparatusaccording to any one of claims 1-6 wherein the strain gauge is formed onor in a substrate by selective diffusion or implantation into thesurface of the substrate.
 9. A strain sensing apparatus according toclaim 7 or claim 8 wherein the temperature gauge is formed before orafter the strain gauge by selective deposition onto the substrate orstrain gauge.
 10. A strain sensing apparatus according to claim 7 orclaim 8 wherein the temperature gauge is formed before or after thestrain gauge by selective diffusion into the substrate or into a layeroverlying the strain gauge.
 11. A strain sensing apparatus according toany one of claims 1-10 wherein the strain gauge is formed, at least inpart, of a material comprising platinum and tungsten.
 12. A strainsensing apparatus according to any one of claims 1-10 wherein the straingauge is formed, at least in part, of a material comprising silicon. 13.A strain sensing apparatus according to any one of claims 1-10 whereinthe temperature gauge is formed, at least in part, of a materialcomprising nickel or platinum.
 14. Apparatus for outputting atemperature-compensated strain measurement, including: a strain sensingapparatus according to any one of claims 1-13, and means for correctingthe output from the strain gauge of said strain sensing apparatus usingthe output from the temperature gauge of said strain sensing apparatus.15. A method of measuring strain using apparatus according to any one ofclaims 1-14, wherein the output from the strain gauge is corrected togive a temperature-compensated strain output according to the outputfrom the temperature gauge.
 16. A method according to claim 15 whereinthe strain is measured during a thermal transient applied to the strainsensing apparatus.